Build Docker image for each PR

Currently we build images for `latest`, release tags, and `latest-snapshot` on main.
We should also build **ephemeral Docker images for each pull request** to allow previewing and testing before merge.

### Proposal
- On every PR build, push Docker images to ECR:
- `neptune/graph-explorer:pr-`
- `neptune/graph-explorer:pr--`
- `pr-` always points to the most recent build for that PR.
- `pr--` is immutable and tied to a specific commit.

### Acceptance Criteria
- [ ] GitHub Actions workflow builds images on `pull_request` events.
- [ ] Images are tagged with both PR number and commit SHA.
- [ ] Images are pushed to ECR successfully.
- [ ] PR images do not interfere with `latest` or release builds.

Start with the existing GitHub Actions workflow that builds the latest, release-tagged, and latest-snapshot Docker images. Trace how those images are tagged and pushed to ECR, then define the pull_request build path around the stated PR-number and commit-SHA tags. Done means both tags are pushed for each PR without changing latest or release builds.

Written by the indexing model from the issue text.